Mew was secretly added to Pokémon Red and Green as the hidden 151st species.
Mew was a late addition to the original games and was not normally obtainable through standard gameplay. Its existence became a subject of rumors among players, helping sustain interest in the games after release. The creature was eventually distributed through special events and promotions rather than ordinary encounters.
The Pokémon was numbered 151 in the original Pokédex, after the 150 obtainable or discoverable species associated with the first-generation adventure. Mewtwo, despite its name and story connection to Mew, is a separate Pokémon and is numbered 150.
A 1996 CoroCoro Comic promotion offered selected readers the chance to receive Mew. Only 20 winners were chosen in that initial lottery, making the Pokémon unusually mysterious and rare. Mew's hidden status became one of the earliest examples of Pokémon using secrets and limited events to encourage player discussion.